Water-resources data for Colorado for water year 2003 consist of records of stage, discharge, and water quality of streams; stage, contents, and water-quality of lakes and reservoirs; meteorological data; and water levels and water quality of wells and springs. This report (volumes 1 and 2) contains discharge records for 329 gaging stations, stage and contents of 19 lakes and reservoirs, discharge measurements for 1 partial-record low-flow station and 1 miscellaneous site, peak-flow information for 23 crest-stage partial-record stations, water-quality data for 128 gaging stations and 8 lakes and reservoirs, supplemental water-quality data for 182 gaged sites, water-quality data for 61 miscellaneous sites and 15 observation wells, water levels for 3 observation wells; and meteorological data for 62 sites. Three pertinent stations operated by bordering States also are included in this report. The records were collected and computed by the Water Resources Discipline of the U.S. Geological Survey under the direction of W.F. Horak, District Chief. These data represent that part of the National Water Data System collected by the U.S. Geological Survey and by cooperating State and Federal agencies.